MOS Transistor with Gate Trench Adjacent to Drain Extension Field Insulation

ABSTRACT

An integrated circuit containing an MOS transistor with a trenched gate abutting an isolation dielectric layer over a drift region. The body well and source diffused region overlap the bottom surface of the gate trench. An integrated circuit containing an MOS transistor with a first trenched gate abutting an isolation dielectric layer over a drift region, and a second trenched gate located over a heavily doped buried layer. The buried layer is the same conductivity type as the drift region. A process of forming an integrated circuit containing an MOS transistor, which includes an isolation dielectric layer over a drift region of a drain of the transistor, and a gate formed in a gate trench which abuts the isolation dielectric layer. The gate trench is formed by removing substrate material adjacent to the isolation dielectric layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The following co-pending patent application is related and herebyincorporated by reference: U.S. patent Provisional application Ser. No.12/101,762 (Texas Instruments docket number TI-65072), filed Apr. 11,2008.

FIELD OF THE INVENTION

This invention relates to the field of integrated circuits. Moreparticularly, this invention relates to drain extended MOS (DEMOS)transistors and laterally diffused MOS (LDMOS) transistors in integratedcircuits.

BACKGROUND OF THE INVENTION

It may be desirable to increase on-state current densities in drainextended MOS (DEMOS) transistors and laterally diffused MOS (LDMOS)transistors in integrated circuits, without sacrificing such parametersas breakdown voltage, hot carrier injection and safe operating area. Inparticular, it may be desirable to reduce electrical resistanceassociated with confined semiconductor regions between isolationdielectric layers and source regions in DEMOS and LDMOS transistors.

SUMMARY OF THE INVENTION

The following presents a simplified summary in order to provide a basicunderstanding of one or more aspects of the invention. This summary isnot an extensive overview of the invention, and is neither intended toidentify key or critical elements of the invention, nor to delineate thescope thereof. Rather, the primary purpose of the summary is to presentsome concepts of the invention in a simplified form as a prelude to amore detailed description that is presented later.

An integrated circuit containing an MOS transistor is provided in whichthe transistor includes a gate formed in a gate trench abutting anisolation dielectric layer over a drift region. A body well overlaps aportion of the bottom surface of the gate trench, and a source diffusedregion overlaps a portion of the bottom gate trench surface within thebody well overlap.

An integrated circuit containing an MOS transistor is provided in whichthe transistor includes a gate formed in a first gate trench abutting anisolation dielectric layer over a drift region, and in a second gatetrench opposite the isolation dielectric layer and located over aheavily doped buried layer. The buried layer is the same conductivitytype as the drift region.

A process of forming an integrated circuit containing an MOS transistoris provided, which includes an isolation dielectric layer over a driftregion of a drain of the transistor, and a gate formed in a gate trenchwhich abuts the isolation dielectric layer. The gate trench is formed byremoving substrate material adjacent to the isolation dielectric layer.

DESCRIPTION OF THE VIEWS OF THE DRAWING

FIG. 1A through FIG. 1H are cross-sections of an integrated circuitcontaining a transistor formed according to an embodiment of the instantinvention, depicted in successive stages of fabrication.

FIG. 2A through FIG. 2D are cross-sections of integrated circuitscontaining transistors formed according to embodiments of the instantinvention, with different configurations of gate and source regions.

FIG. 3 and FIG. 4 are cross-sections of integrated circuits containingtransistors formed according to embodiments of the instant invention, inwhich body wells are formed prior to and after, respectively, formationof gates of the transistors.

FIG. 5A and FIG. 5B, and FIG. 6A and FIG. 6B depict two exemplaryprocess sequences for forming field plates.

FIG. 7A through FIG. 7D, FIG. 8A through FIG. 8D, FIG. 9A through FIG.9D, FIG. 10A through FIG. 10D, and FIG. 11A through FIG. 11D are topviews of embodiments of configurations of gate trenches and isolationdielectric layers.

FIG. 12A and FIG. 12B are top views of two realizations of theembodiment discussed in reference to FIG. 11A through FIG. 11D.

FIG. 13 is a top view of a realization of the embodiment discussed inreference to FIG. 2G.

DETAILED DESCRIPTION

The present invention is described with reference to the attachedfigures, wherein like reference numerals are used throughout the figuresto designate similar or equivalent elements. The figures are not drawnto scale and they are provided merely to illustrate the invention.Several aspects of the invention are described below with reference toexample applications for illustration. It should be understood thatnumerous specific details, relationships, and methods are set forth toprovide a full understanding of the invention. One skilled in therelevant art, however, will readily recognize that the invention can bepracticed without one or more of the specific details or with othermethods. In other instances, well-known structures or operations are notshown in detail to avoid obscuring the invention. The present inventionis not limited by the illustrated ordering of acts or events, as someacts may occur in different orders and/or concurrently with other actsor events. Furthermore, not all illustrated acts or events are requiredto implement a methodology in accordance with the present invention.

FIG. 1A through FIG. 1H are cross-sections of an integrated circuitcontaining a transistor formed according to an embodiment of the instantinvention, depicted in successive stages of fabrication. The transistormay be a DEMOS or LDMOS transistor. Referring to FIG. 1A, the integratedcircuit (1000) is formed in and on a substrate (1002) which is commonlya single crystal silicon wafer, but may be a silicon-on-insulator (SOI)wafer, a hybrid orientation technology (HOT) wafer with regions ofdifferent crystal orientations, or other material appropriate forfabrication of the IC (1000). An optional heavily doped buried layer(1004) may be formed in the substrate (1002) under an area designatedfor the transistor (1006), for example by ion implanting dopants intothe substrate (1002) followed by epitaxial semiconductor film growth onan existing top surface of the substrate (1002). The optional heavilydoped buried layer (1004), if formed, has a same conductivity type assource and drain regions of the transistor (1006). A drift region (1008)is formed in the substrate (1002) between the source and drain regions.The drift region (1008) has a same conductivity type as the source anddrain regions. The drift region (1008) is formed above the optionalheavily doped buried layer (1004), if formed. A body well (1010) isformed in the substrate (1002) under a channel area of the transistor(1006). The body well may be formed by ion implanting dopants into thesubstrate (1002). A conductivity type of the body well (1010) isopposite the conductivity type of the source and drain regions. Optionalelements of field oxide (1012) may be formed at a top surface of thesubstrate (1002) to isolate the transistor (1006) from other components(not shown) in the integrated circuit (1000).

A layer of isolation dielectric (1014) is formed at the top surface ofthe substrate (1002) over the drift region (1008). The isolationdielectric layer (1014) may be formed concurrently with field oxideelements in the integrated circuit (1000) or may be formed by otherprocess steps. In one embodiment, the isolation dielectric layer (1014)is between 200 and 500 nanometers thick.

A gate trench photoresist pattern (1016) is formed on the top surface ofthe integrated circuit (1000) to define a gate trench area (1018)adjacent to, and possibly overlapping a portion of, the isolationdielectric layer (1014).

Referring to FIG. 1B, a gate trench (1020) is etched in the substrate(1002) in the gate trench area defined by the gate trench photoresistpattern (1016). In one embodiment, a bottom surface of the gate trench(1020) may be deeper than a bottom surface of the isolation dielectriclayer (1014), as depicted in FIG. 1B. In another embodiment, the bottomsurface of the gate trench (1020) may be substantially even with thebottom surface of the isolation dielectric layer (1014). In a furtherembodiment, the bottom surface of the gate trench (1020) may beshallower than the bottom surface of the isolation dielectric layer(1014). The gate trench (1020) may be etched using a fluorine containingplasma during an anisotropic reactive ion etch (RIE) process, or usingan isotropic etch process such as immersing the substrate (1002) in acholine solution, or a combination of an anisotropic etch process and anisotropic etch process. Bottom edges of the gate trench (1020) may berounded by any of various processes such as wet etching. After the gatetrench (1020) is formed, the gate trench photoresist pattern (1016) isremoved, commonly by exposing the IC (1000) to an oxygen containingplasma, followed by a wet cleanup to remove any organic residue from thetop surface of the IC (1000).

Referring to FIG. 1C, a gate dielectric layer (1022) is formed onexposed surfaces of the substrate (1002) in and adjacent to the gatetrench (1020). The gate dielectric layer (1022) may be one or morelayers of silicon dioxide (SiO₂), silicon oxy-nitride (SiON), aluminumoxide (Al₂O₃), aluminum oxy-nitride (AlON), hafnium oxide (HfO), hafniumsilicate (HfSiO), hafnium silicon oxy-nitride (HfSiON), zirconium oxide(ZrO), zirconium silicate (ZrSiO), zirconium silicon oxy-nitride(ZrSiON), a combination of the aforementioned materials, or otherinsulating material. The gate dielectric layer (1022) may includenitrogen as a result of exposure to a nitrogen containing plasma or anitrogen containing ambient gas at temperatures between 50 C and 800 C.The gate dielectric layer (1022) is typically between 1 and 10nanometers thick. The gate dielectric layer (1022) may be formed by anyof a variety of gate dielectric formation processes, for example thermaloxidation, plasma nitridation of an oxide layer, and/or dielectricmaterial deposition by atomic layer deposition (ALD). An additional gatedielectric layer (not shown) may be grown on exposed surfaces of thesubstrate (1002) in the drain region of the transistor (1006).

Referring to FIG. 1D, a gate layer (1024) is formed on an existing topsurface of the integrated circuit (1000), including exposed surfaces ofthe gate dielectric layer (1022) in the gate trench (1020). The gatelayer (1024) may be formed of polycrystalline silicon, commonly known aspolysilicon, or other conductive material such as titanium nitride. Inone embodiment, a gate photoresist pattern (1026) is formed on a topsurface of the gate layer (1024) to define an area for a gate of thetransistor (1006). The area for the gate defined by the gate photoresistpattern (1026) overlaps the gate trench (1020).

Referring to FIG. 1E, a gate (1028) is formed by removing unwantedmaterial from the gate layer. In embodiments using gate photoresistpatterns, unwanted gate layer material may be removed using etchprocesses such as RIE, to leave the gate (1028) as depicted in FIG. 1E.In some embodiments, the gate (1028) may overlap a portion of theisolation dielectric layer (1014). After the gate (1028) is formed, thegate photoresist pattern (1026) is removed, as described in reference tothe gate trench photoresist pattern depicted in FIG. 1B. In otherembodiments, gate layer material may be removed from the top surface ofthe substrate (1002) using planar removal processes such as chemicalmechanical polish (CMP) or isotropic etchback, to form the gate (1028)in the gate trench, so that a top surface of the gate (1028) issubstantially coplanar with the top surface of the substrate (1002)adjacent to the gate trench.

Referring to FIG. 1F, an optional source lightly doped drain (LDD)implanted region (1030) is formed in the source region of the transistor(1006), by ion implanting dopants into the substrate (1002) in thesource region. A conductivity type of the source LDD implanted region(1030) is the same as the drain region and the drift region (1008). Anoptional drain LDD implanted region (not shown) may be formed in thedrain region concurrently with the source LDD implanted region (1030). Atotal dose of dopants in the source LDD implanted region (1030) may bebetween 3·10¹⁴ and 3·10¹⁵ atoms/cm². The source LDD implanted region(1030) extends from the top surface of the substrate (1002) to a depthbetween 10 nanometers and the depth of the gate trench.

Referring to FIG. 1G, optional sidewall spacers (1032) may be formed onlateral surfaces of the gate (1028). The sidewall spacers (1032) may beformed by deposition of one or more conformal layers of silicon nitrideand/or silicon dioxide on a top and lateral surfaces of the gate (1028)and the top surface of the substrate (1002), followed by removal of theconformal layer material from the top surface of the gate (1028) and thetop surface of the substrate (1002) by known anisotropic etchingmethods, leaving the conformal layer material on the lateral surfaces ofthe gate (1028). A source implanted region (1034) is formed byimplanting dopants into the substrate (1002) in the source regionadjacent to the sidewall spacers (1032), if formed, or the gate (1028)if no sidewall spacers are present. A dose of the source implantedregion (1034) may be between 3·10¹⁴ and 1·10¹⁶ atoms/cm². The sourceimplanted region (1034) extends from the top surface of the substrate(1002) to a depth between 100 and 1000 nanometers. A conductivity typeof the source implanted region (1034) is the same as the source LDDimplanted region (1030).

A drain implanted region (1036) is formed by ion implanting dopants intothe substrate (1002) in the drain region proximate to the isolationdielectric layer (1014). A conductivity type of the source implantedregion (1034) is the same as the source implanted region (1032). In someembodiments, the drain implanted region (1036) is formed concurrentlywith the source implanted region (1034).

Referring to FIG. 1H, an anneal operation is performed which activate aportion of the dopants implanted into the source LDD implanted region,the source implanted region and the drain implanted region to form asource diffused region (1038) and a drain diffused region (1040).Optional layers of metal silicide (not shown) may be formed on topsurfaces of the source diffused region (1038), the drain diffused region(1040), or the gate (1028).

A pre-metal dielectric (PMD) layer (1042) is formed on an existing topsurface of the integrated circuit (1000). The PMD layer (1042) istypically a dielectric layer stack including a PMD liner, a PMD mainlayer, and an optional PMD cap layer. The PMD liner is commonly siliconnitride or silicon dioxide, 10 to 100 nanometers thick, deposited byplasma enhanced chemical vapor deposition (PECVD) on the integratedcircuit (1000). The PMD main layer is commonly a layer of silicondioxide, phospho-silicate glass (PSG) or boro-phospho-silicate glass(BPSG), commonly 100 to 1000 nanometers thick, deposited by PECVD on atop surface of the PMD liner, and sometimes leveled by achemical-mechanical polish (CMP) process. The optional PMD cap layer iscommonly 10 to 100 nanometers of a hard material such as siliconnitride, silicon carbide nitride or silicon carbide, formed on a topsurface of the PMD main layer. Source and drain contacts (1044) areformed in the PMD layer (1042) to make electrical connections to thesource diffused region (1038) and the drain diffused region (1040). Thecontacts (1044) may be formed by defining contact areas on a top surfaceof the PMD layer (1042) with a contact photoresist pattern (not shown),etching contact holes in the contact areas by removing PMD layermaterial using known etching methods to expose the source diffusedregion (1038) and the drain diffused region (1040), and filling thecontact holes with a contact liner metal, such as titanium, and acontact fill metal, typically tungsten, followed by removal of thecontact fill metal from the top surface of the PMD layer (1042) usingknown etching and/or CMP methods.

During operation of the transistor (1006) depicted in FIG. 1H, the gate(1028) is biased so as to form an inversion layer under the gatedielectric layer (1022), and possibly adjacent to lateral surfaces ofthe gate dielectric layer (1022), in the body well (1010). Forming thetransistor (1006) with the gate (1028) in the gate trench allows currentto flow directly from the inversion layer to the drift region (1008)without passing through a confined semiconductor region, which mayadvantageously increase an on-state current in the transistor (1006). Inembodiments in which the gate (1028) overlaps the isolation dielectriclayer (1012), an electric field at an interface between the isolationdielectric layer (1012) and the drift region 1008) may be advantageouslyreduced, allowing operation of the transistor (1006) at a higher drainpotential. In other embodiments, a separate field plate may be formed ona to surface of the isolation dielectric layer (1012) to enable areduction of an electric field at an interface between the isolationdielectric layer (1012) and the drift region (1008).

FIG. 2A through FIG. 2G are cross-sections of integrated circuitscontaining transistors formed according to embodiments of the instantinvention, with different configurations of gate and source regions. Theembodiments depicted in FIG. 2A through FIG. 2G are meant to beexemplary, but not comprehensive. Referring to FIG. 2A, a firstintegrated circuit (2000) is formed in and on a first substrate (2002)as described in reference to FIG. 1A. An optional first heavily dopedburied layer (2004) may be formed under the first transistor (2006) andan optional first field oxide element (2008) may be formed to isolatethe first transistor (2006) from other components in the firstintegrated circuit (2000), as described in reference to FIG. 1A. Thefirst transistor (2006) includes a first drift region (2010), a firstbody well (2012) and a first isolation dielectric layer (2014) formedover the first drift region (2010), as described in reference to FIG.1A.

A first gate trench (2016) is formed in the first substrate (2002)adjacent to the first isolation dielectric layer (2014), as described inreference to FIG. 1B and FIG. 1C. A bottom surface of the first gatetrench (2016) is substantially coplanar with a bottom surface of thefirst isolation dielectric layer (2014).

A first gate dielectric layer (2018) is formed on exposed surfaces ofthe first substrate (2002) in and adjacent to the first gate trench(2016), as described in reference to FIG. 1C. A first gate (2020) isformed in the first gate trench (2016) on the first gate dielectriclayer (2018), as described in reference to FIG. 1D and FIG. 1E. In theinstant embodiment, the first gate (2020) may be formed conformally inthe first gate trench (2016) as depicted in FIG. 2A. A stress layer(2022) is formed on a top surface of the first gate (2020) which mayprovide a desired level of tensile or compressive stress to a channelregion in the first body well (2012) adjacent to the first gatedielectric layer (2018). The stress layer (2022) may be formed, forexample, of metal silicide, silicon nitride, or other material with adesired stress and compatibility with fabrication of the firstintegrated circuit (2000). A first sidewall spacer (2024) may be formedon lateral surfaces of the first gate (2020), as described in referenceto FIG. 1G. A first source diffused region (2026) is formed in the bodywell (2012) as described in reference to FIG. F through FIG. 1H.

The first transistor (2006) is operated as described in reference toFIG. 1H. Forming the first gate trench (2016) so that its bottom surfaceis substantially coplanar with the bottom surface of the first isolationdielectric layer (2014) may provide reduced series resistance comparedto a configuration in which a gate trench bottom surface is shallowerthan an isolation dielectric layer bottom surface, and higher gate-drainbreakdown voltage than a configuration in which a gate trench bottomsurface is deeper than an isolation dielectric layer bottom surface.

Referring to FIG. 2B, a second integrated circuit (2028) is formed inand on a second substrate (2030) as described in reference to FIG. 1A.An optional second heavily doped buried layer (2032) may be formed underthe second transistor (2034) and an optional second field oxide element(2036) may be formed to isolate the second transistor (2034) from othercomponents in the second integrated circuit (2028), as described inreference to FIG. 1A. The second transistor (2034) includes a seconddrift region (2038), a second body well (2040) and a second isolationdielectric layer (2042) formed over the second drift region (2038), asdescribed in reference to FIG. 1A.

A second gate trench (2044) is formed in the second substrate (2030)adjacent to the second isolation dielectric layer (2042), as describedin reference to FIG. 1B and FIG. 1C. A bottom surface of the second gatetrench (2044) is shallower than a bottom surface of the second isolationdielectric layer (2042).

A second gate dielectric layer (2046) is formed on exposed surfaces ofthe second substrate (2030) in and adjacent to the second gate trench(2044), as described in reference to FIG. 1C. A second gate (2048) isformed in the second gate trench (2044) on the second gate dielectriclayer (2046), as described in reference to FIG. 1D and FIG. 1E. A secondsidewall spacer (2050) may be formed on lateral surfaces of the secondgate (2048), as described in reference to FIG. 1G. A second sourcediffused region (2052) is formed in the body well (2040) as described inreference to FIG. F through FIG. 1H.

The second transistor (2034) is operated as described in reference toFIG. 1H. Forming the second gate trench (2044) so that its bottomsurface is shallower than the bottom surface of the second isolationdielectric layer (2042) may provide increased breakdown voltage comparedto configurations in which the gate trench bottom surface is deeper thanthe isolation dielectric layer bottom surface, and increased gate trenchetch process latitude compared to a configuration in which a gate trenchbottom surface is substantially coplanar with an isolation dielectriclayer bottom surface.

Referring to FIG. 2C, a third integrated circuit (2054) is formed in andon a third substrate (2056) as described in reference to FIG. 1A. Anoptional third heavily doped buried layer (2058) may be formed under thethird transistor (2060) and an optional third field oxide element (2062)may be formed to isolate the third transistor (2060) from othercomponents in the third integrated circuit (2054), as described inreference to FIG. 1A. The third transistor (2060) includes a third driftregion (2064), a third body well (2066) and a third isolation dielectriclayer (2068) formed over the third drift region (2064), as described inreference to FIG. 1A.

A third gate trench (2070) is formed in the third substrate (2056)adjacent to the third isolation dielectric layer (2068), as described inreference to FIG. 1B and FIG. 1C. A bottom surface of the third gatetrench (2070) is substantially coplanar with a bottom surface of thethird isolation dielectric layer (2068).

A third gate dielectric layer (2072) is formed on exposed surfaces ofthe third substrate (2056) in and adjacent to the third gate trench(2070), as described in reference to FIG. 1C. A third gate (2074) isformed in the third gate trench (2070) on the third gate dielectriclayer (2072), as described in reference to FIG. 1D and FIG. 1E. In theinstant embodiment, the third gate (2074) has a lateral overlap of asource region of the third transistor (2060) of less than 20 nanometers.A third sidewall spacer (2076) may be formed on lateral surfaces of thethird gate (2074), as described in reference to FIG. 1G. A third sourcediffused region (2078) is formed in the body well (2066) as described inreference to FIG. F through FIG. 1H, such that the third source diffusedregion (2078) contacts a vertical face of the third gate dielectriclayer (2072). In the instant embodiment, the third body well (2066) mayabut the vertical face of the third gate dielectric layer (2072) and notoverlap the bottom surface of the third gate trench (2070), as depictedin FIG. 2C.

The third transistor (2060) is operated as described in reference toFIG. 1H. Forming the third gate (2074) so that the third source diffusedregion (2078) contacts the vertical face of the third gate dielectriclayer (2072) may provide reduced inversion channel length compared to aconfiguration in which a source diffused region is laterally separatedfrom a vertical face of a gate dielectric layer, which mayadvantageously provide a higher on-state current of the third transistor(2060).

Referring to FIG. 2D, a fourth integrated circuit (2080) is formed inand on a fourth substrate (2082) as described in reference to FIG. 1A.An optional fourth heavily doped buried layer (2084) may be formed underthe fourth transistor (2086) and an optional fourth field oxide element(2088) may be formed to isolate the fourth transistor (2086) from othercomponents in the fourth integrated circuit (2080), as described inreference to FIG. 1A. The fourth transistor (2086) includes a fourthdrift region (2090), a fourth body well (2092) and a fourth isolationdielectric layer (2094) formed over the fourth drift region (2090), asdescribed in reference to FIG. 1A.

A fourth gate trench (2096) is formed in the fourth substrate (2082)adjacent to the fourth isolation dielectric layer (2094), as describedin reference to FIG. 1B and FIG. 1C. A bottom surface of the fourth gatetrench (2096) is deeper than a bottom surface of the fourth isolationdielectric layer (2094).

A fourth gate dielectric layer (2098) is formed on exposed surfaces ofthe fourth substrate (2082) in and adjacent to the fourth gate trench(2096), as described in reference to FIG. 1C. A fourth gate (2100) isformed in the fourth gate trench (2096) on the fourth gate dielectriclayer (2098), as described in reference to FIG. 1D and FIG. 1E. In theinstant embodiment, the fourth gate (2100) has a lateral overlap of asource region of the fourth transistor (2086) of less than 20nanometers. A fourth sidewall spacer (2102) may be formed on lateralsurfaces of the fourth gate (2100), as described in reference to FIG.1G. A fourth source diffused region (2104) is formed in the body well(2092) as described in reference to FIG. F through FIG. 1H, such thatthe fourth source diffused region (2104) contacts a vertical face of thefourth gate dielectric layer (2098).

The fourth transistor (2086) is operated as described in reference toFIG. 1H. Forming the fourth gate (2100) so that the fourth sourcediffused region (2104) contacts the vertical face of the fourth gatedielectric layer (2098) may provide reduced inversion channel lengthcompared to a configuration in which a source diffused region islaterally separated from a vertical face of a gate dielectric layer,which may advantageously provide a higher on-state current of the fourthtransistor (2086).

Referring to FIG. 2E, a fifth integrated circuit (2106) is formed in andon a fifth substrate (2108) as described in reference to FIG. 1A. Anoptional fifth heavily doped buried layer (2110) may be formed under thefifth transistor (2112) and an optional fifth field oxide element (2114)may be formed to isolate the fifth transistor (2112) from othercomponents in the fifth integrated circuit (2106), as described inreference to FIG. 1A. The fifth transistor (2112) includes a fifth driftregion (2116), a fifth body well (2118) and a fifth isolation dielectriclayer (2120) formed over the fifth drift region (2116), as described inreference to FIG. 1A.

A fifth gate trench (2122) is formed in the fifth substrate (2108)adjacent to the fifth isolation dielectric layer (2120), as described inreference to FIG. 1B and FIG. 1C. A bottom surface of the fifth gatetrench (2122) may be deeper than a bottom surface of the fifth isolationdielectric layer (2120) as depicted in FIG. 2E, may be substantiallycoplanar with the bottom surface of the fifth isolation dielectric layer(2120), or may be shallower than the bottom surface of the fifthisolation dielectric layer (2120).

In the instant embodiment, a doping density of the fifth body well(2118) at lateral surfaces and the bottom surface of the fifth gatetrench (2122) is substantially constant. The substantially constantdoping density may be formed, for example, by forming a diffusionbarrier such as silicon nitride on an existing top surface of the fifthintegrated circuit (2106) including the fifth gate trench (2122),removing the diffusion barrier from an area defined for a channel of thefifth transistor (2112), and performing a thermal diffusion of dopantsthrough exposed surfaces of the fifth gate trench (2122) into adjacentregions of the fifth substrate (2108).

A fifth source diffused region (2124) is formed in the fifth body well(2118). In the instant embodiment, the fifth source diffused region(2124) extends from the top surface of the fifth substrate (2108) to adepth below the fifth gate trench (2122). In one realization of theinstant embodiment, a lateral distance from a boundary of the fifthsource diffused region (2124) to a boundary of the fifth body well(2118) is substantially constant on lateral surfaces and the bottomsurface of the fifth gate trench (2122), so as to provide asubstantially constant channel length across the transistor (2112). Inone realization, a portion of the fifth source diffused region (2124)may be formed by thermally diffusing dopants through exposed surfaces ofthe fifth gate trench (2122) into adjacent regions of the fifthsubstrate (2108) using a similar process to that described for formingthe fifth body well (2118).

A fifth gate dielectric layer (2126) is formed on exposed surfaces ofthe fifth substrate (2108) in and adjacent to the fifth gate trench(2122), as described in reference to FIG. 1C. A fifth gate (2128) isformed in the fifth gate trench (2122) on the fifth gate dielectriclayer (2126), as described in reference to FIG. 1D and FIG. 1E. A fifthsidewall spacer (2130) may be formed on lateral surfaces of the fifthgate (2128), as described in reference to FIG. 1G.

The fifth transistor (2112) is operated as described in reference toFIG. 1H. Forming the fifth body well (2118) to have a substantiallyconstant doping density at the lateral surfaces and bottom surface ofthe fifth gate trench (2122) may provide an increased channel width withmore uniform threshold compared to a configuration in which a sourcediffused region is less deep than a corresponding gate trench, which mayadvantageously provide a higher on-state current of the fifth transistor(2112).

Referring to FIG. 2F, a sixth integrated circuit (2132) is formed in andon a sixth substrate (2134) as described in reference to FIG. 1A. Anoptional sixth heavily doped buried layer (2136) may be formed under thesixth transistor (2138) and an optional sixth field oxide element (2140)may be formed to isolate the sixth transistor (2138) from othercomponents in the sixth integrated circuit (2132), as described inreference to FIG. 1A. The sixth transistor (2138) includes a sixth driftregion (2142), a sixth body well (2144) and a sixth isolation dielectriclayer (2146) formed over the sixth drift region (2142), as described inreference to FIG. 1A. In the instant embodiment, a top surface of thesixth body well (2144) is located below a top surface of the sixthsubstrate (2134).

A sixth gate trench (2148) is formed in the sixth substrate (2134)adjacent to the sixth isolation dielectric layer (2146), as described inreference to FIG. 1B and FIG. 1C. A bottom surface of the sixth gatetrench (2148) may be deeper than a bottom surface of the sixth isolationdielectric layer (2146) as depicted in FIG. 2F, may be substantiallycoplanar with the bottom surface of the sixth isolation dielectric layer(2146), or may be shallower than the bottom surface of the sixthisolation dielectric layer (2146).

A sixth source diffused region (2150) is formed in the sixth body well(2144). In the instant embodiment, the sixth source diffused region(2150) extends from the top surface of the sixth substrate (2134) to adepth below the sixth gate trench (2148). In one realization of theinstant embodiment, a portion of the sixth source diffused region (2150)may be formed by thermally diffusing dopants through exposed surfaces ofthe sixth gate trench (2148) into adjacent regions of the sixthsubstrate (2134) using a similar process to that described for formingthe fifth source diffused region (2124) in reference to FIG. 2E.

A sixth gate dielectric layer (2152) is formed on exposed surfaces ofthe sixth substrate (2134) in and adjacent to the sixth gate trench(2148), as described in reference to FIG. 1C. A sixth gate (2154) isformed in the sixth gate trench (2148) on the sixth gate dielectriclayer (2152), as described in reference to FIG. 1D and FIG. 1E. A sixthsidewall spacer (2156) may be formed on lateral surfaces of the sixthgate (2154), as described in reference to FIG. 1G.

The sixth transistor (2138) is operated as described in reference toFIG. 1H. Forming the source diffused region (2150) to extend to thebottom surface of the sixth gate trench (2148) may provide an increasedchannel width compared to a configuration in which a source diffusedregion is less deep than a corresponding gate trench, which mayadvantageously provide a higher on-state current of the sixth transistor(2138).

Referring to FIG. 2G, a seventh integrated circuit (2158) is formed inand on a seventh substrate (2160) as described in reference to FIG. 1A.A seventh heavily doped buried layer (2162) is formed under the seventhtransistor (2164) as described in reference to FIG. 1A. The seventhtransistor (2158) includes a seventh drift region (2166), a seventh bodywell (2168) and a seventh isolation dielectric layer (2170) formed overthe seventh drift region (2166), as described in reference to FIG. 1A. Aboundary of the seventh body well (2168) intersects regions for gatetrenches so as to form channel regions for the seventh transistor(2158). In the instant embodiment, a doping density of the seventhheavily doped buried layer (2162) is more than 10 times a doping densityof the seventh drift region (2166).

A seventh gate trench set includes a first trench (2172) formed in theseventh substrate (2160) adjacent to the seventh isolation dielectriclayer (2170), as described in reference to FIG. 1B and FIG. 1C. A bottomsurface of the first trench (2172) may be deeper than a bottom surfaceof the seventh isolation dielectric layer (2170) as depicted in FIG. 2G,may be substantially coplanar with the bottom surface of the seventhisolation dielectric layer (2170), or may be shallower than the bottomsurface of the seventh isolation dielectric layer (2170). The seventhgate trench set includes a second trench (2174) formed in the seventhsubstrate (2160) proximate to the first trench (2172) opposite theseventh isolation dielectric layer (2170). A bottom surface of thesecond trench (2174) may be deeper than the bottom surface of theseventh isolation dielectric layer (2170) as depicted in FIG. 2G, may besubstantially coplanar with the bottom surface of the seventh isolationdielectric layer (2170), or may be shallower than the bottom surface ofthe seventh isolation dielectric layer (2170). The seventh gate trenchset may further include an optional third trench (2176) formed in theseventh substrate (2160) proximate to the second trench (2174) oppositethe first trench (2172). In one realization of the instant embodiment,the first trench (2172), second trench (2174) and third trench (2176)may be formed concurrently. In an alternate realization, the trenches(2170, 2172, 2174) may be formed separately.

A seventh gate dielectric layer (2178) is formed on exposed surfaces ofthe first trench (2172), second trench (2174) and third trench (2176) ifpresent, as described in reference to FIG. 1C. A seventh gate (2180) isformed in the first trench (2172), second trench (2174) and third trench(2176) if present, on the seventh gate dielectric layer (2178), asdescribed in reference to FIG. 1D and FIG. 1E. In one realization of theinstant embodiment, the seventh gate (2180) may be planarized so that atop surface of the seventh gate (2180) is substantially coplanar with atop surface of the seventh substrate (2160) as depicted in FIG. 2G. Inan alternate realization, the seventh gate (2180) may overlap the topsurface of the seventh substrate (2160).

A seventh source diffused region (2182) is formed in the seventh bodywell (2168) as described in reference to FIG. F through FIG. 1H, suchthat the seventh source diffused region (2182) contacts vertical facesof the seventh gate dielectric layer (2178) at the first trench (2172),second trench (2174) and third trench (2176) if present.

The seventh transistor (2164) is operated as described in reference toFIG. 1H. Current may flow from the seventh source diffused region (2182)through a channel region adjacent to the first trench (2172) laterallyinto the seventh drift region (2166). Current may further flow from theseventh source diffused region (2182) through a channel region adjacentto the second trench (2174) vertically into the seventh heavily dopedburied layer (2162). If the third trench (2176) is present, current mayfurther flow from the seventh source diffused region (2182) through achannel region adjacent to the third trench (2176) vertically into theseventh heavily doped buried layer (2162). Forming the second trench(2174) and third trench (2176) if present may provide additional currentpaths compared to a configuration with one gate trench, which mayadvantageously provide a higher on-state current of the seventhtransistor (2164).

A body well in a transistor formed according to an embodiment of theinstant invention may be formed prior to forming a gate of thetransistor, or may be formed after forming the gate of the transistor,depending on desired levels of on-state current. FIG. 3 and FIG. 4 arecross-sections of integrated circuits containing transistors formedaccording to embodiments of the instant invention, in which body wellsare formed prior to and after, respectively, formation of gates of thetransistors. Referring to FIG. 3, an integrated circuit (3000) is formedin and on a substrate (3002) as described in reference to FIG. 1A. Anisolation dielectric layer (3004) is formed over a drift region (3006)in a transistor (3008) which is formed according to an embodiment of theinstant invention. A body well photoresist pattern (3010) is formed on atop surface of the integrated circuit (3000) to define an area forimplanting body well dopants. A body well ion implant process (3012) isperformed which implants the body well dopants into the substrate (3002)to form a body well implanted region (3014). In subsequent steps, a gatetrench will be formed, of which a bottom surface will intersect aboundary of the body well (3014). Forming the body well implanted region(3014) prior to forming a gate trench in the transistor (3006) mayprovide a longer inversion channel length, which may advantageouslyprovide a desired level of precision or repeatability of an on-statecurrent in the transistor (3006).

Referring to FIG. 4, an integrated circuit (4000) is formed in and on asubstrate (4002) as described in reference to FIG. 1A. An isolationdielectric layer (4004) is formed over a drift region (4006) in atransistor (4008) which is formed according to an embodiment of theinstant invention. A gate trench is formed abutting or overlapping theisolation dielectric layer (4004) as described in reference to FIG. 1Aand FIG. 1B. A gate dielectric layer (4010) is formed on exposedsurfaces of the gate trench, as described in reference to FIG. 1C. Agate (4012) is formed in the gate trench on exposed surfaces of the gatedielectric layer (4010), as described in reference to FIG. 1D and FIG.1E, so that the gate (4012) does not overlap a top surface of thesubstrate (40020 in a source area of the transistor (4008), as describedin reference to FIG. 2D. A body well ion implant process (4014) isperformed which implants the body well dopants into the substrate (4002)to form a body well implanted region (4016). The gate (4012) blocks thebody well dopants so that a boundary of the body well implanted region(4016) is substantially aligned with a vertical face of the gatedielectric layer (4010). An inversion channel length may be limited to adifference between a depth of the gate trench and a depth of asubsequent source diffused region. Forming the body well implantedregion (4016) after forming the gate trench in the transistor (4008) mayprovide a shorter inversion channel length, which may advantageouslyprovide a desired high level of an on-state current in the transistor(4008).

Field plates on isolation dielectric layers in transistors formedaccording embodiments of the instant invention may provide advantageousreductions in electric fields at interfaces between the isolationdielectric layers and abutting drift regions, as described in referenceto FIG. 1H. Field plates may be formed by various process sequences, inaddition to the process sequence described in reference to FIG. 1Athrough FIG. 1H. FIG. 5A and FIG. 5B, and FIG. 6A and FIG. 6B depict twoexemplary process sequences for forming field plates. Referring to FIG.5A, an integrated circuit (5000) is formed in and on a substrate (5002)as described in reference to FIG. 1A. The integrated circuit (5000)contains an area for a transistor (5004) formed according to anembodiment of the instant invention. Optional elements of field oxide(5006) may be formed at a top surface of the substrate (5002) to isolatethe transistor (5004) from other components in the integrated circuit(5000) as described in reference to FIG. 1A. A drift region (5008) isformed in the transistor (5004) and an isolation dielectric layer (5010)is formed at the top surface of the substrate (5002) over the driftregion (5008) as described in reference to FIG. 1A. A body well (5012)is formed in the transistor (5004) as described in reference to FIG. 1A.A gate trench (5014) is formed in the transistor (5004) in the substrate(5002) abutting, and possibly overlapping, the isolation dielectriclayer (5010) as described in reference to FIG. 1A and FIG. 1B. A gatedielectric layer (5016) is formed on exposed surfaces of the gate trench(5014) as described in reference to FIG. 1C. A gate layer (5018) isformed in the gate trench (5014) and on a top surface of the isolationdielectric layer (5010) as described in reference to FIG. 1D. A gatephotoresist pattern (5020) is formed on a top surface of the gate layer(5018) to define a gate area (5022) over the gate trench (5014) for agate of the transistor, as described in reference to FIG. 1D, and todefine a field plate area (5024) over the isolation dielectric layer(5010) for a field plate. In the instant embodiment, the gate area(5022) does not abut or overlap the field plate area (5024).

Referring to FIG. 5B, unwanted gate layer material is removed, forexample by etching processes described in reference to FIG. 1E to form agate (5026) in the gate trench (5014) and a field plate (5028) on theisolation dielectric layer (5010). The gate photoresist pattern isremoved after the gate (5026) and field plate (5028) are formed, asdescribed in reference to FIG. 1E. Forming the field plate (5028) to beelectrically separated from the gate (5026) enables the field plate(5028) to be biased separately from the gate (5026), which mayadvantageously increase a maximum drain voltage of the transistor(5004). Forming the field plate (5028) concurrently with the gate (5026)advantageously reduces a fabrication cost and complexity of theintegrated circuit (5000) compared to embodiments in which field platesand gates are not formed concurrently.

Referring to FIG. 6A, an integrated circuit (6000) is formed in and on asubstrate (6002) as described in reference to FIG. 1A. The integratedcircuit (6000) contains an area for a transistor (6004) formed accordingto an embodiment of the instant invention. Optional elements of fieldoxide (6006) may be formed at a top surface of the substrate (6002) toisolate the transistor (6004) from other components in the integratedcircuit (6000) as described in reference to FIG. 1A. A drift region(6008) is formed in the transistor (6004) and an isolation dielectriclayer (6010) is formed at the top surface of the substrate (6002) overthe drift region (6008) as described in reference to FIG. 1A. A bodywell (6012) is formed in the transistor (6004) as described in referenceto FIG. 1A. A gate trench (6014) is formed in the transistor (6004) inthe substrate (6002) abutting, and possibly overlapping, the isolationdielectric layer (6010) as described in reference to FIG. 1A and FIG.1B. A gate dielectric layer (6016) is formed on exposed surfaces of thegate trench (6014) as described in reference to FIG. 1C. A gate (6018)is formed in the gate trench (6014) on the gate dielectric layer (6016)as described in reference to FIG. 1D and FIG. 1E. In one realization ofthe instant embodiment, the gate (6018) is planarized so that a topsurface of the gate (6018) is substantially coplanar with a top surfaceof the isolation dielectric layer (6010) as depicted in FIG. 6A.

A field plate layer (6020) is formed on an existing top surface of theintegrated circuit (6000). The field plate layer (6020) may be one ofvarious forms of silicon, a metallic material such as tungsten ortitanium nitride, other conductive material, or a combination of thesematerials. A field plate photoresist pattern (6022) is formed on a topsurface of the field plate layer (6020) to define the area for the fieldplate. In the instant embodiment, the area for the field plate does notabut or overlap the gate (6018).

Referring to FIG. 6B, the field plate (6024) is formed by removingunwanted material from the field plate layer, for example by an RIEprocess. The field plate photoresist pattern is removed after the fieldplate is formed, for example by exposing the integrated circuit (6000)to an oxygen containing plasma, followed by a wet cleanup to remove anyorganic residue from the top surface of the integrated circuit (6000).Forming the field plate (6024) to be electrically separated from thegate (6018) enables the field plate (6024) to be biased separately fromthe gate (6018), which may advantageously increase a maximum drainvoltage of the transistor (6004). Forming the field plate (6024) using aseparate layer from the gate (6018) enables the field plate (6024) to belocated closer to the gate (6018), which may allow reduction of anelectric field in the drift region (6008) at an interface with theisolation dielectric layer (6010), which may advantageously increase amaximum drain voltage of the transistor (6004). In an alternaterealization of the instant embodiment, the field plate (6024) may beformed prior to forming the gate (6018).

Transistors formed according to the instant invention may include morethan one gate trench and/or more than one isolation dielectric layer.FIG. 7A through FIG. 7D, FIG. 8A through FIG. 8D, FIG. 9A through FIG.9D, FIG. 10A through FIG. 10D, FIG. 11A through FIG. 11D, and FIG. 12Aand FIG. 12B are top views of embodiments of configurations of gatetrenches and isolation dielectric layers. Field oxide elements, bodywells and field plates are not shown in FIG. 7A through FIG. 7D, FIG. 8Athrough FIG. 8D, FIG. 9A through FIG. 9D, FIG. 10A through FIG. 10D,FIG. 11A through FIG. 11D, and FIG. 12A and FIG. 12B for clarity.

FIG. 7A through FIG. 7D are top views of a transistor (7000) formedaccording to an embodiment containing one gate trench and one isolationdielectric layer, formed according to the instant invention, depicted insuccessive stages of fabrication. Referring to FIG. 7A, a singleisolation dielectric layer (7002) is formed in an integrated circuit(not shown). Referring to FIG. 7B, a single gate trench (7004) is formedabutting and possibly overlapping the isolation dielectric layer (7002)as described in reference to FIG. 1A and FIG. 1B. A width of the gatetrench (7004) may be less than a width of the isolation dielectric layer(7002) as depicted in FIG. 7B, or may be wider in other embodiments.Referring to FIG. 7C, a gate (7006) is formed in the gate trench (7004)as described in reference to FIG. 1D and FIG. 1E. Referring to FIG. 7D,a source diffused region (7008) is formed adjacent to the gate trench(7004) as described in reference to FIG. 1F through FIG. 1H. A draindiffused region (7010) is formed adjacent to the isolation dielectriclayer (7002) opposite the gate trench (7004) as described in referenceto FIG. 1F through FIG. 1H. Forming the transistor (7000) in theconfiguration depicted in FIG. 7D may enable the advantages described inreference to FIG. 1H.

FIG. 8A through FIG. 8D are top views of a transistor (8000) formedaccording to an embodiment containing multiple gate trenches configuredin parallel and one isolation dielectric layer, formed according to theinstant invention, depicted in successive stages of fabrication.Referring to FIG. 8A, a single isolation dielectric layer (8002) isformed in an integrated circuit (not shown). Referring to FIG. 8B,multiple gate trenches (8004) are formed abutting and possiblyoverlapping the isolation dielectric layer (8002) as described inreference to FIG. 1A and FIG. 1B. A total width of the gate trenches(8004) may be less than a width of the isolation dielectric layer (8002)as depicted in FIG. 8B, or may be wider in other embodiments. Referringto FIG. 8C, a gate (8006) is formed in the gate trenches (8004) asdescribed in reference to FIG. 1D and FIG. 1E. Referring to FIG. 8D, asource diffused region (8008) is formed adjacent to the gate trenches(8004) as described in reference to FIG. 1F through FIG. 1H. A draindiffused region (8010) is formed adjacent to the isolation dielectriclayer (8002) opposite the gate trenches (8004) as described in referenceto FIG. 1F through FIG. 1H. Forming multiple gate trenches (8004) in thetransistor (8000) as depicted in FIG. 8D may provide an increasedeffective width of an inversion channel due to formation of inversionlayers adjacent to lateral surfaces of the gate trenches (8004), whichmay advantageously provide an increased on-state current in thetransistor (8000).

FIG. 9A through FIG. 9D are top views of a transistor (9000) formedaccording to an embodiment containing a single gate trench and multipleisolation dielectric layers configured in parallel, formed according tothe instant invention, depicted in successive stages of fabrication.Referring to FIG. 9A, multiple isolation dielectric layers (9002) areformed in an integrated circuit (not shown). A drift region of thetransistor (9000) may extend to regions between the isolation dielectriclayers (9002). Referring to FIG. 9B, a single gate trench (9004) isformed abutting and possibly overlapping the isolation dielectric layers(9002) as described in reference to FIG. 1A and FIG. 1B. A width of thegate trench (9004) may be less than a total width of the isolationdielectric layers (9002) as depicted in FIG. 9B, or may be wider inother embodiments. Referring to FIG. 9C, a gate (9006) is formed in thegate trench (9004) as described in reference to FIG. 1D and FIG. 1E.Referring to FIG. 9D, a source diffused region (9008) is formed adjacentto the gate trench (9004) as described in reference to FIG. 1F throughFIG. 1H. A drain diffused region (9010) is formed adjacent to theisolation dielectric layers (9002) opposite the gate trench (9004) asdescribed in reference to FIG. 1F through FIG. 1H. Forming thetransistor (9000) in the configuration depicted in FIG. 9D may provide areduced series resistance of the drift region due to additional driftarea in regions between the isolation dielectric layers (9002), whichmay advantageously provide an increased on-state current in thetransistor (9000).

FIG. 10A through FIG. 10D are top views of a transistor (10000) formedaccording to an embodiment containing multiple gate trenches andmultiple isolation dielectric layers, both configured in parallel,formed according to the instant invention, depicted in successive stagesof fabrication. Referring to FIG. 10A, multiple isolation dielectriclayers (10002) are formed in an integrated circuit (not shown). A driftregion of the transistor (10000) may extend to regions between theisolation dielectric layers (10002). Referring to FIG. 10B, multiplegate trenches (10004) are formed abutting and possibly overlapping theisolation dielectric layers (10002) as described in reference to FIG. 1Aand FIG. 1B. A total width of the gate trenches (10004) may be less thana total width of the isolation dielectric layers (10002) as depicted inFIG. 10B, or may be wider in other embodiments. Referring to FIG. 10C, agate (10006) is formed in the gate trenches (10004) as described inreference to FIG. 1D and FIG. 1E. Referring to FIG. 10D, a sourcediffused region (10008) is formed adjacent to the gate trenches (10004)as described in reference to FIG. 1F through FIG. 1H. A drain diffusedregion (10010) is formed adjacent to the isolation dielectric layers(10002) opposite the gate trench (10004) as described in reference toFIG. 1F through FIG. 1H. Forming multiple gate trenches (10004) in thetransistor (10000) as depicted in FIG. 10C may provide an increasedeffective width of an inversion channel due to formation of inversionlayers adjacent to lateral surfaces of the gate trenches (10004), whichmay advantageously provide an increased on-state current in thetransistor (10000). Additionally, forming the transistor (10000) in theconfiguration depicted in FIG. 10D may provide a reduced seriesresistance of the drift region due to additional drift area in regionsbetween the isolation dielectric layers (10002), which may alsoadvantageously provide a further increase of on-state current in thetransistor (10000).

FIG. 11A through FIG. 11D are top views of a transistor (11000) formedaccording to an embodiment containing multiple gate trenches andmultiple isolation dielectric layers, both configured in parallel, inwhich each gate trench is aligned with an isolation dielectric layer,formed according to the instant invention, depicted in successive stagesof fabrication. Referring to FIG. 11A, multiple isolation dielectriclayers (11002) are formed in an integrated circuit (not shown). A driftregion of the transistor (11000) may extend to regions between theisolation dielectric layers (11002). Referring to FIG. 11B, multiplegate trenches (11004) are formed abutting and possibly overlapping theisolation dielectric layers (11002) as described in reference to FIG. 1Aand FIG. 1B, so that each gate trench (11004) is aligned with aisolation dielectric layer (11002). In one embodiment, a width of eachgate trench (11004) may be substantially equal to a width of eachcorresponding isolation dielectric layer (11002) as depicted in FIG.11B. In another embodiment, a width of each gate trench (11004) may begreater than a width of each corresponding isolation dielectric layer(11002). In yet another embodiment, a width of each gate trench (11004)may be less than a width of each corresponding isolation dielectriclayer (11002). Referring to FIG. 11C, a gate (11006) is formed in thegate trenches (11004) as described in reference to FIG. 1D and FIG. 1E.Referring to FIG. 11D, a source diffused region (11008) is formedadjacent to the gate trenches (11004) as described in reference to FIG.1F through FIG. 1H. A drain diffused region (11010) is formed adjacentto the isolation dielectric layers (11002) opposite the gate trench(11004) as described in reference to FIG. 1F through FIG. 1H. Formingmultiple gate trenches (11004) aligned with isolation dielectric layers(11002) in the transistor (11000) as depicted in FIG. 11D may provide anincreased effective width of an inversion channel due to formation ofinversion layers adjacent to lateral surfaces of the gate trenches(11004) which is efficiently coupled to the drift region, which mayadvantageously provide an increased on-state current in the transistor(11000) compared to other embodiments.

FIG. 12A and FIG. 12B are top views of two realizations of theembodiment discussed in reference to FIG. 11A through FIG. 11D.Referring to FIG. 12A, a first transistor (12000) includes multiplefirst isolation dielectric layers (12002) formed in a first integratedcircuit (not shown). Multiple first gate trenches (11004) are formedabutting and possibly overlapping the first isolation dielectric layers(11002). In the instant realization, each first gate trench (11004) iswider than a corresponding isolation dielectric layer (12002). A firstgate (12006) is formed in the trenches (12004). A first source diffusedregion (12008) is formed adjacent to the gate trenches (12004) and afirst drain diffused region (12010) is formed adjacent to the isolationdielectric layers (12002) opposite the gate trenches (12004). Formingmultiple gate trenches (12004) wider than isolation dielectric layers(12002) in the transistor (12000) as depicted in FIG. 12A may provide anefficient coupling of channel current to a drift region under andbetween the isolation dielectric layers (12002), which mayadvantageously provide an increased on-state current in the transistor(12000) compared to other embodiments.

Referring to FIG. 12B, a second transistor (12012) includes multiplesecond isolation dielectric layers (12014) formed in a second integratedcircuit (not shown). Multiple second gate trenches (11016) are formedabutting and possibly overlapping the second isolation dielectric layers(11014). In the instant realization, each second gate trench (11016) isnarrower than a corresponding isolation dielectric layer (12014). Asecond gate (12018) is formed in the trenches (12016). A second sourcediffused region (12020) is formed adjacent to the gate trenches (12016)and a second drain diffused region (12022) is formed adjacent to theisolation dielectric layers (12014) opposite the gate trenches (12016).Forming multiple gate trenches (12004) narrower than isolationdielectric layers (12002) in the transistor (12000) as depicted in FIG.12A may provide an increased effective channel width for a size of thetransistor (12000), which may advantageously provide an increasedon-state current in the transistor (12000) compared to other embodimentsof a substantially equal size.

FIG. 13 is a top view of a realization of the embodiment discussed inreference to FIG. 2G. A transistor (13000) includes multiple isolationdielectric layers (13002) formed in an integrated circuit (not shown). Aburied layer (not shown) is formed in a substrate of the integratedcircuit under the transistor (13000) as described in reference to FIG.2G. A set of first gate trenches (13004) is formed abutting and possiblyoverlapping the isolation dielectric layers (13002). In the instantrealization, each instance of the first gate trenches (13004) is alignedwith a corresponding instance of the isolation dielectric layers(13002). A set of second gate trenches (13006) is formed proximate tothe first gate trenches (13004) opposite the isolation dielectric layers(13002). In the instant realization, each instance of the second gatetrenches (13006) is aligned with a corresponding instance of the firstgate trenches (13004). In alternate realizations, the second gatetrenches (13006) may not be aligned with the first gate trenches(13004). A gate (13008) is formed in the first gate trenches (13004) andthe second gate trenches (13006). A source diffused region (13010) isformed adjacent to the first gate trenches (13004) and the second gatetrenches (13006). The source diffused region (13010) may optionally belocated on both sides of the second gate trenches (13006) as depicted inFIG. 13. A drain diffused region (13012) is formed adjacent to theisolation dielectric layers (13002) opposite the first gate trenches(13004).

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only and not limitation. Numerous changes to the disclosedembodiments can be made in accordance with the disclosure herein withoutdeparting from the spirit or scope of the invention. Thus, the breadthand scope of the present invention should not be limited by any of theabove described embodiments. Rather, the scope of the invention shouldbe defined in accordance with the following claims and theirequivalents.

1. An integrated circuit containing an MOS transistor, said integratedcircuit comprising: a semiconductor substrate; a drift region formed insaid substrate, said drift region having a first conductivity type, suchthat said drift region extends to a top surface of said substrate; anisolation dielectric layer formed at said top surface of said substrateover said drift region; a gate trench formed in said substrate, suchthat said gate trench abuts said isolation dielectric layer; a body wellformed in said substrate adjacent to said drift region, such that saidbody well overlaps a first portion of a bottom surface of said gatetrench, and such that said body well has an opposite conductivity typefrom said drift region; a gate dielectric layer formed on exposedsurfaces of said substrate in said gate trench; a gate formed on saidgate dielectric layer in said gate trench; and a source diffused regionformed in said body well, such that said source diffused region abutssaid gate dielectric layer in said gate trench and overlaps a secondportion of said bottom surface of said gate trench, said second portionof said bottom surface of said gate trench being within said firstportion of said bottom surface of said gate trench, and such that saidsource diffused region has a same conductivity type as said driftregion.
 2. The integrated circuit of claim 1, further including: asecond isolation dielectric layer formed at said top surface of saidsubstrate over said drift region, such that said second isolationdielectric layer is separate from said first isolation dielectric layer;and a second gate trench formed in said substrate such that: said secondgate trench is separate from said first gate trench; said second gatetrench abuts said second isolation dielectric layer; said body welloverlaps a first portion of a bottom surface of said second gate trench;said gate dielectric layer is formed on exposed surfaces of saidsubstrate in said second gate trench; said gate is formed on said gatedielectric layer in said second gate trench; and said source diffusedregion abuts said gate dielectric layer in said second gate trench andoverlaps a second portion of said bottom surface of said second gatetrench, said second portion of said bottom surface of said second gatetrench being within said first portion of said bottom surface of saidsecond gate trench.
 3. The integrated circuit of claim 2, in which saidgate overlaps said substrate between said first gate trench and saidsecond gate trench.
 4. The integrated circuit of claim 1, in which alateral distance from a boundary of said source diffused region to aboundary of said body well is substantially constant on lateral surfacesand said bottom surface of said gate trench.
 5. The integrated circuitof claim 1, in which a depth of said gate trench is less than a depth ofsaid isolation dielectric layer.
 6. The integrated circuit of claim 1,further including a field plate formed on a top surface of saidisolation dielectric layer.
 7. An integrated circuit containing an MOStransistor, said integrated circuit comprising: a semiconductorsubstrate; a drift region formed in said substrate, said drift regionhaving a first conductivity type and extending to a top surface of saidsubstrate; an isolation dielectric layer formed at said top surface ofsaid substrate over said drift region; a first gate trench formed insaid substrate, such that said first gate trench abuts said isolationdielectric layer; a second gate trench formed in said substrateproximate to said first gate trench opposite from said isolationdielectric layer, such that said second gate trench is separate fromsaid first gate trench; a body well formed in said substrate adjacent tosaid drift region, such that said body well abuts said first gate trenchand said second gate trench, and such that said body well has anopposite conductivity type from said drift region; a buried layer formedin said substrate under said drift region, and under said first gatetrench and said second gate trench, such that said buried layer has asame conductivity type as said drift region, and such that a dopingdensity of said buried layer is more than 10 times a doping density ofsaid drift region; a gate dielectric layer formed on exposed surfaces ofsaid substrate in said first gate trench and in said second gate trench;a gate formed on said gate dielectric layer in said first gate trenchand in said second gate trench; and a source diffused region formed insaid body well, such that said source diffused region abuts said gatedielectric layer in said first gate trench and in said second gatetrench, and such that said source diffused region has a sameconductivity type as said drift region.
 8. The integrated circuit ofclaim 7, further including: a second isolation dielectric layer formedat said top surface of said substrate over said drift region, such thatsaid second isolation dielectric layer does not overlap a gate region ofsaid transistor, and such that said second isolation dielectric layer isseparate from said first isolation dielectric layer; a third gate trenchformed in said substrate, such that: said third gate trench is separatefrom said first gate trench and said second gate trench; said third gatetrench abuts said second isolation dielectric layer; said body wellabuts said third gate trench; said gate dielectric layer is formed onexposed surfaces of said substrate in said third gate trench; said gateis formed on said gate dielectric layer in said third gate trench; andsaid source diffused region abuts said gate dielectric layer in saidthird gate trench; and a fourth gate trench formed in said substrate,such that: said fourth gate trench is separate from said first gatetrench, said second gate trench and said third gate trench; said fourthgate trench abuts said second isolation dielectric layer; said body wellabuts said fourth gate trench; said gate dielectric layer is formed onexposed surfaces of said substrate in said fourth gate trench; said gateis formed on said gate dielectric layer in said fourth gate trench; andsaid source diffused region abuts said gate dielectric layer in saidsecond gate trench.
 9. The integrated circuit of claim 8, in which saidgate overlaps said substrate between said first gate trench and saidthird gate trench, and overlaps said substrate between said second gatetrench and said fourth gate trench.
 10. The integrated circuit of claim7, in which a depth of said first gate trench is less than a depth ofsaid isolation dielectric layer.
 11. The integrated circuit of claim 7,in which: said body well overlaps a first portion of a bottom surface ofsaid first gate trench; and said source diffused region overlaps asecond portion of said bottom surface of said first gate trench, saidsecond portion of said bottom surface of said first gate trench beingwithin said first portion of said bottom surface of said first gatetrench.
 12. The integrated circuit of claim 7, further including astress layer formed on a top surface of said gate.
 13. A process offorming an integrated circuit containing an MOS transistor, comprisingthe steps: providing a semiconductor substrate; forming a drift regionin said substrate, such that said drift region has a first conductivitytype and so that said drift region extends to a top surface of saidsubstrate; forming an isolation dielectric layer at said top surface ofsaid substrate over said drift region, such that said isolationdielectric layer does not overlap a gate region of said transistor;forming a gate trench in said substrate so that said gate trench abutssaid isolation dielectric layer; forming a body well in said substrateadjacent to said drift region, such that said body well has an oppositeconductivity type from said drift region; forming a gate dielectriclayer on exposed surfaces of said substrate in said gate trench; forminga gate on said gate dielectric layer in said gate trench; and forming asource diffused region in said body well, such that said source diffusedregion abuts said gate dielectric layer in said gate trench, and suchthat said source diffused region has a same conductivity type as saiddrift region.
 14. The process of claim 13, further including the steps:forming a second isolation dielectric layer at said top surface of saidsubstrate over said drift region, such that said isolation dielectriclayer does not overlap a gate region of said transistor, and such thatsaid second isolation dielectric layer is separate from said firstisolation dielectric layer; and forming a second gate trench in saidsubstrate, such that: said second gate trench is separate from saidfirst gate trench; said second gate trench abuts said second isolationdielectric layer; said gate dielectric layer is formed on exposedsurfaces of said substrate in said second gate trench; said gate isformed on said gate dielectric layer in said second gate trench; andsaid source diffused region abuts said gate dielectric layer in saidsecond gate trench.
 15. The process of claim 14, in which said processof forming said gate is performed so that said gate overlaps saidsubstrate between said first gate trench and said second gate trench.16. The process of claim 13, in which said step of forming said gatetrench is performed so that a depth of said gate trench is less than adepth of said isolation dielectric layer.
 17. The process of claim 13,in which: said process of forming a body well is performed so that saidbody well overlaps a first portion of a bottom surface of said gatetrench; and said process of forming said source diffused region isperformed so that said source diffused region overlaps a second portionof said bottom surface of said gate trench, said second portion of saidbottom surface of said gate trench being within said first portion ofsaid bottom surface of said gate trench.
 18. The process of claim 17, inwhich said process of forming said body well and said process of formingsaid source diffused region are performed so that a lateral distancefrom a boundary of said source diffused region to a boundary of saidbody well is substantially constant on lateral surfaces and said bottomsurface of said gate trench.
 19. The process of claim 13, furtherincluding the steps: forming a second gate trench in said substrateproximate to said first gate trench opposite from said isolationdielectric layer, such that: said second gate trench is separate fromsaid first gate trench; said body well abuts said second gate trench;said gate dielectric layer is formed on exposed surfaces of saidsubstrate in said second gate trench; said gate is formed on said gatedielectric layer in said second gate trench; and said source diffusedregion abuts said gate dielectric layer in said second gate trench; andforming a buried layer in said substrate under said drift region, andunder said first gate trench and said second gate trench, such that saidburied layer has a same conductivity type as said drift region, and suchthat a doping density of said buried layer is more than 10 times adoping density of said drift region.
 20. The process of claim 13,further including the step of forming a field plate on a top surface ofsaid isolation dielectric layer.